Avaya 8800 Planning And Engineering, Network Design page 144

SPBM design guidelines
Figure 60: SPBM implementation options
A—IP Shortcut
IP Shortcuts forward standard IP packets over IS-IS. This option enables you to forward IP over the
SPBM core, which is a simpler method than traditional IP routing or MPLS. SPBM nodes propagate
Layer 3 reachability as "leaf" information in the IS-IS LSPs using Extended IP reachability TLV 135,
which contains routing information such as neighbors and locally configured subnets. SPBM nodes
receiving the reachability information use this information to populate the routes to the announcing
nodes. All TLVs announced in the IS-IS LSPs are grafted onto the shortest path tree (SPT) as leaf
nodes.
An IP route lookup is only required once where the source BEB uses the GRT to identify the BEB
closest to the destination subnet. All other nodes perform standard Ethernet switching based on the
existing SPT. This allows for end to end IP-over-Ethernet forwarding without the need for ARP,
flooding, or reverse learning. Because BCB SPBM nodes only forward on the MAC addresses that
comprise the B-MAC header, and since unknown TLVs in IS-IS are relayed to the next hop but
ignored locally, SPBM BCB nodes need not be aware of IP subnets to forward IP traffic. Only BEBs
generate and receive Extended IP reachability TLV to build routing table in GRT; BCBs just relay the
TLV to the next hop based on SPT. In fact, the Extended IP reachabilty TLV is ignored on BCBs.
With IP Shortcuts there are only two IP routing hops (ingress BEB and egress BEB) as the SPBM
backbone acts as a virtualized switching backplane.
IP Shortcuts do not require any I-SID configuration. However, IP must be enabled on IS-IS and the
IS-IS source address is configured to matched a circuitless/loopback IP address.
In the figure above, node 8600G is acting as a BCB for the service, and has no IP configuration
whatsoever.
June 2016 Planning and Engineering — Network Design
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